In general, a technology for grinding a round of an edge of a semiconductor wafer includes vertical grinding and helical grinding. The vertical grinding technology rotates a grinding wheel having a groove on a level with a surface of a semiconductor wafer, contacts a surface of the groove with an edge of the semiconductor wafer and grinds the edge of the semiconductor wafer using shape and roughness of the groove. The helical grinding technology rotates a grinding wheel having a groove at a predetermined angle relative to a surface of a semiconductor wafer, contacts a surface of the groove with an edge of the semiconductor wafer and grinds the edge of the semiconductor wafer.
In grinding an edge of a semiconductor wafer using the above-mentioned technology, a grinding wheel has a groove, of which shape corresponds to that of the edge of the semiconductor wafer, in conformity with the predetermined quality specifications.
The grinding wheel, in particular, the groove is made of a metal bond or a resin bond.
A grinding wheel having a metal bond groove has excellent wear resistance, and thus, although the number of times of wafer edge grinding increases, the grinding wheel suffers a little change in shape of the groove caused by wear and eliminates the need to true or dress the groove during wafer edge grinding. However, the grinding wheel having the metal bond groove forms a damaged layer of a predetermined depth from the surface of the wafer edge and generates a fine scratch such as a wheel mark on the surface of the wafer edge, and thus does not meet customer demands for wafer surface quality.
And, a grinding wheel having a resin bond groove guarantees a good grinding quality, but has a slow grinding speed and a poor wear resistance of the groove, and consequently suffers a change in shape of the groove during wafer edge grinding. Thus, the resin bond groove needs truing or dressing in a predetermined cycle. In particular, in the case that a helical grinding technology is applied, wafer edge grinding is complicated, a diameter of the grinding wheel is limited due to a wheel balance problem and life of a spindle is reduced.
Here, ‘truing’ means, when the shape of a groove of a grinding wheel is changed, restoring the shape of the groove using a truing tool (hereinafter referred to as a truer, and a conventional truer has similar thickness and diameter to a wafer) having an edge of a shape corresponding to a standard shape of the groove. ‘Dressing’ means removing grinding swarf that may be loaded in between exposed grits of a truer, and removing chips caught in exposed pores using a diamond dresser to expose new grits to the surface, thereby restoring a grinding performance.
A conventional wafer edge grinding process performs double grinding that grinds (rough-grinds) a considerable amount of edge of a semiconductor wafer using a grinding wheel having a metal bond groove and then grinds (fine-grinds) the edge of the semiconductor wafer using a grinding wheel having a resin bond groove to remove a fine scratch such as a wheel mark. This process can simultaneously make up for grinding quality reduction pointed out as a disadvantage of the metal bond groove, and life reduction caused by a low wear resistance, pointed out as a disadvantage of the resin bond groove.
A conventional wafer edge grinding apparatus is described with reference to FIGS. 1 to 5. FIG. 1 is a view illustrating a notch and a round of a wafer edge.
Referring to FIGS. 2 to 5, the conventional wafer edge grinding apparatus 10 includes a chuck operating unit 20 for fixing and rotating a wafer W, a grinding wheel 30 for grinding an edge of the wafer W, and a truer S for truing grooves 32′ and 34′ of the grinding wheel 30. At this time, the grinding wheel 30 includes rough-grinding wheels 31 and 33 having metal bond grooves for rough-grinding a notch and a round of the wafer W, and fine-grinding wheels 32 and 34 having resin bond grooves for fine-grinding a notch and a round of the wafer W. Specifically, the rough-grinding wheels 31 and 33 include a round rough-grinding wheel 31 for rough-grinding a round of the wafer W, and a notch rough-grinding wheel 33 for rough-grinding a notch of the wafer W. The fine-grinding wheels 32 and 34 include a notch fine-grinding wheel 32 for fine-grinding a notch of the wafer W, and a round fine-grinding wheel 34 for fine-grinding a round of the wafer W. At this time, the round fine-grinding wheel 34 is slanted at a predetermined angle, and thus it is also referred to as a helical wheel.
The grinding wheel 30 rotates in the direction equal or opposite to a rotation direction of the wafer W, and contacts with the edge of the wafer W to grind the edge of the wafer W using shape and roughness of the groove.
Meanwhile, because the grooves 32′ and 34′ of the notch fine-grinding wheel 32 and the round fine-grinding wheel 34 are worn down after a predetermined time passes by or grinding a predetermined number of wafers, the worn grooves 32′ and 34′ should be trued. The truing is made by the truer S having shape and dimension corresponding to thickness and diameter of the wafer W.
A wafer edge grinding process using the grinding apparatus 10 and a truing process using the truer S are described as follows.
According to the wafer edge grinding process, first, center, thickness and notch of a wafer W are measured. Next, the wafer W is loaded on a rotatable chuck 21 (mounted on a processing stage), and a round of the wafer W is rough-ground. Subsequently, a notch of the wafer W is rough-ground and fine-ground, and the round of the wafer W is fine-ground. Finally, the wafer W is unloaded.
According to the truing process, center and thickness of the truer S are measured. Next, the truer S is mounted on the chuck 21, and compensated by a truer compensating tool embedded in the round rough-grinding wheel 31. The groove 32′ of the notch fine-grinding wheel 32 or the groove 34′ of the round fine-grinding wheel 34 is selectively trued by the compensated truer S.
As shown in FIG. 4, the notch fine-grinding wheel 32 has a plurality of grooves 32′ on the surface thereof. The grooves 32′ fine-grind the wafer W. The grooves 32′ are trued by the truer S. The notch fine-grinding wheel 32 grinds upper and lower slanted surfaces of the edge of the wafer W separately, and uses an air bearing with a spindle mounting the wheel 32. Due to these characteristics, when the number of times of wafer edge processing and truing exceeds a predetermined number, a wear unbalance phenomenon occurs to the notch fine-grinding wheel 32. The wear unbalance phenomenon is resulted from an increase in grinding amount by a volume indicated by diagonal lines (see FIG. 4(a)) when grinding the lower slanted surface of the edge of the wafer W. An alternate processing is used to minimize the wear unbalance of an equipment itself. The alternate processing grinds a lower slanted surface of an edge of a first wafer W, and then grinds an upper slanted surface (see FIG. 4(b)) of an edge of a second wafer W. The alternate processing alternates a processing sequence to balance a grinding amount, thereby reducing wear unbalance. However, in spite of use of the alternate processing, a wear unbalance phenomenon still occurs due to characteristics of a bearing with a spindle. As a result, as shown in FIG. 6, a notch of the first wafer W and a notch of the second wafer W are formed in different shapes. That is, there is a predetermined difference in grinding amount between a wafer of an odd number and a wafer of an even number, resulting in wear unbalance.
The round fine-grinding wheel 34 has a spindle mounted at a predetermined angle, for example 8°. When the round fine-grinding wheel 34 or its groove 34′ is replaced by a new one, a new groove has a shape that is not in conformity with the shape quality specification. Thus, after the round fine-grinding wheel 34 or its groove 34′ is replaced by a new one, a new groove should be trued by the truer S. If the new groove is not trued, because an edge of a wafer W is ground by a groove that is not in conformity with the shape quality specification, the edge of the wafer W has a shape that does not meet the shape quality specification as shown in FIG. 7, and consequently the wafer W is regarded as a faulty product. Therefore, after a grinding wheel or its groove is replaced by a new one, a new groove should be trued by the truer S to meet the shape quality specification for wafer edge.
However, as shown in FIG. 8, as the number of times of wafer edge grinding and truing by the truer S increases, the groove 34′ is worn down and a wheel diameter at the groove 34′ is reduced. When a wear amount reaches a predetermined amount (1 mm in radius, 30 times of truings), the use of the corresponding groove 34′ is stopped to prevent an over-grinding phenomenon that the wafer edge is over-ground, and the groove 34′ or the grinding wheel 34 is replaced by a new one. Although there is a small room for grinding due to difference in thickness between the truer S and the wafer W, an over-grinding phenomenon may occur due to a small change in Z-axis or flatness of a wafer or a small change in flatness of a chuck that may be caused by impurities on the surface of the chuck. This is why a grinding wheel or its groove is replaced by a new one.
The over-grinding problem generally comes to the notch fine-grinding wheel 32 and the round fine-grinding wheel 34. The over-grinding is recognized by an edge profiler or a microscope with a scale. The upper and lower bevel values are measured, in the case that the values exceed a predetermined range, it is determined as over-grinding, and a subsequent process is performed, for example the grooves 32′ and 34′ or the grinding wheels 32 and 34 are replaced. Even though a considerable portion of a resin bond groove is available, a grinding wheel is replaced, resulting in life reduction of the grinding wheel.
Meanwhile, the truer S is manufactured by powder-sintering ceramics as a basic material and various indispensable impurities (including diamond particles). The truer S is useful for truing of a groove, however it wears down a portion of the grinding wheels 32 and 34 slightly during truing, resulting in change in grinding dimension of the wafer W. After truing, it requires the time to set the wafer processing conditions.
The fine-grinding wheels 32 and 34 are manufactured by sintering diamond particles and a thermosetting resin such as a phenol resin or a polyamide resin. The thermosetting resin acts as a bond. During wafer edge grinding, the round fine-grinding wheel 34 is rotated at a high speed, for example, at a linear velocity of about 5000 m/min (about 30000 rpm to 40000 rpm), and the notch fine-grinding wheel 32 is rotated at a high speed, for example, at a linear velocity of about 500 m/min (about 150000 rpm). At this time, a friction heat is not removed due to outer environmental cause, resulting in a burning phenomenon. The groove of the grinding wheel is burned and hardened due to characteristics of a thermosetting resin. The burned portion is not removed by the truer S. And, if a wafer is ground by the burned grinding wheel, the diamond particles cannot work on the wafer due to the hardened resin bond groove, and consequently the wafer edge is not ground. That is, the burned grinding wheel cannot be restored or used due to a material of the truer and impossibility of wafer edge grinding, and thus the grinding wheel should be replaced. As mentioned above, this leads to life reduction of the grinding wheel.